Thanks, Assaf. Do you have the option to take a look at the raw HTTP request that is sent? It could be that the UrlFetchApp API handles POST requests differently than GETs and does not include the Content-Type header or something else. If you have the option to review the actual HTTP request that is really sent, that would make it easier to troubleshoot.

URLFetchApp doesn’t support “Content-Type” headers for GET requests. This is correct, as the Content-Type header denotes the content type of the request body, which is unsupported for GET. The closest “correct” header would be “Accept” for the expected content type of the response body.

TestRail needs to remove the requirement or also allow the header in “Accept”

I’m having the same problem. Content-Type is not a valid header for get requests and several http libraries forbid it. After 2 years, can we get this check removed? It’s putting a hold to several integration projects we are working on.